Translational Diffusion of Linear and 3-Arm-Star Polystyrenes in Semidilute Solutions of Linear Poly(vinyl methyl ether)

The technique of dynamic light scattering from isorefractive ternary solutions has been used to investigate the translational diffusion behavior of linear and 3-arm-star polymers in linear polymer matrices. Diffusion coefficients have been obtained for four polystyrene (PS) samples present in trace amounts in solutions of poly (vinyl methyl ether) (PVME) in o-fluorotoluene over the range 0.001-0.1 g/mL in PVME concentration. The high molecular weight of the PVME sample, 1.3 X 10⁶, guarantees that these concentrations extend well into the entangled regime. For PS with molecular weights around 4 X 10⁵, a 3-arm star diffuses slightly more rapidly than its linear counterpart. However, when the PS molecular weight exceeds 1 X 10⁶, a 3-arm star diffuses much less rapidly than its linear counterpart at the higher matrix concentrations. These data are interpreted as evidence for the importance of topology in determining diffusion rates for polymers in concentrated solutions. While this observation is consistent with the reptation mechanism, it is also apparent that reptation cannot dominate the diffusion process until the diffusing molecules are thoroughly entangled with the matrix.